Method of producing mixed powder for powder metallurgy, method of producing sintered body, and sintered body

ABSTRACT

A method of producing a mixed powder for powder metallurgy comprises: mixing an iron-based powder with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder; heat-treating the raw material mixed powder to cause Mo and Cu to diffusionally adhere to a surface of the iron-based powder, to obtain a partially diffusion-alloyed steel powder; and mixing the partially diffusion-alloyed steel powder with a graphite powder, to obtain a mixed powder for powder metallurgy, wherein the iron-based powder has an average particle size of 30 μm to 120 μm, a cuprous oxide powder is used as the Cu-containing powder, and the mixed powder for powder metallurgy has a chemical composition containing Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %, with a balance being Fe and incidental impurities.

TECHNICAL FIELD

This disclosure relates to a method of producing a mixed powder for powder metallurgy, and particularly relates to a method of producing a mixed powder for powder metallurgy that, despite not containing Ni, has properties suitable for the production of high strength sintered parts for vehicles and the like. This disclosure also relates to a method of producing a sintered body, and a sintered body obtained by the production method.

BACKGROUND

Powder metallurgical techniques enable producing parts having complicated shapes in shapes (i.e. near net shapes) extremely close to product shapes, with high dimensional accuracy. The use of powder metallurgical techniques in producing parts thus contributes to significantly lower machining costs. For this reason, powder metallurgical products obtained by powder metallurgical techniques have been used as various mechanical parts in many fields.

Powder metallurgical techniques mainly use iron-based powders. Iron-based powders are categorized into iron powder (e.g. pure iron powder), alloyed steel powder, and the like, depending on the components. Iron-based powders are also categorized into atomized iron powder, reduced iron powder, and the like, based on the production method. In the case of using the categories by the production method, the term “iron powder” has a broad meaning encompassing not only pure iron powder but also alloyed steel powder.

With powder metallurgical techniques, an iron-based powder mentioned above is used to produce a green compact, and the green compact is sintered to produce a sintered body. The green compact is typically produced by mixing an iron-based powder with alloying powders such as a Cu powder and a graphite powder and a lubricant such as stearic acid or lithium stearate to obtain a mixed powder, and then charging the mixed powder into a die and pressing it.

The density of a green compact obtained by a typical powder metallurgical process is about 6.6 Mg/m³ to 7.1 Mg/m³. The green compact is then sintered to obtain a sintered body. The sintered body is further subjected to optional sizing and machining work, to obtain a powder metallurgical part (component). In the case where higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.

Increases in strength of powder metallurgical parts have been strongly requested recently, for reductions in size and weight of parts. There has been particularly strong demand for strengthening iron-based powder parts (iron-based sintered bodies) made from iron-based powders.

As iron-based powders, the following powders each obtained by adding alloying elements to a raw material powder (pure iron powder) are mainly known:

(1) a mixed powder obtained by adding each alloying element powder to a pure iron powder;

(2) a pre-alloyed steel powder obtained by completely alloying a pure iron powder with each alloying element; and

(3) a partially diffusion-alloyed steel powder (also referred to as “composite alloyed steel powder”) obtained by causing each alloying element powder to partially diffusionally adhere to the surface of a pure iron powder or a pre-alloyed steel powder.

The mixed powder (1) is advantageous in that high compressibility equivalent to that of a pure iron powder is ensured. However, during sintering, each alloying element does not sufficiently diffuse in Fe and the microstructure becomes non-uniform, which can cause a decrease in the strength of the resulting sintered body. Moreover, in the case where Mn, Cr, V, Si, etc. which are more easily oxidizable than Fe are used as alloying elements, they are oxidized during sintering, so that the strength of the resulting sintered body decreases. To suppress such oxidation and lower the oxygen content in the sintered body, it is necessary to strictly control the sintering atmosphere and, in the case of performing carburizing after sintering, the CO₂ concentration and the dew point in the carburizing atmosphere. Hence, the mixed powder (1) has not been used due to its failure to meet the recent requests for strengthening.

With the pre-alloyed steel powder (2), the segregation of alloying elements can be prevented completely, with it being possible to achieve uniform microstructure of the sintered body. This contributes to stable mechanical properties of the sintered body. In addition, even in the case where Mn, Cr, V, Si, etc. are used as alloying elements, lower oxygen content in the sintered body can be achieved by limiting the types and amounts of such alloying elements. However, since the pre-alloyed steel powder is produced by atomizing molten steel, oxidation of the molten steel in the atomizing process and solid solution hardening of the steel powder due to complete alloying tend to occur. This hinders an increase of green density in press forming.

The partially diffusion-alloyed steel powder (3) is produced by adding a metal powder of each alloying element to a pure iron powder or a pre-alloyed steel powder and heating the mixture in a non-oxidizing or reducing atmosphere to partially diffusionally bond the metal powder to the surface of the pure iron powder or pre-alloyed steel powder. The use of the partially diffusion-alloyed steel powder can thus provide the advantages of the iron-based mixed powder (1) and the pre-alloyed steel powder (2), while avoiding the problems of the iron-based mixed powder (1) and the pre-alloyed steel powder (2).

In detail, the partially diffusion-alloyed steel powder achieves both lower oxygen content and high compressibility equivalent to that of a pure iron powder. Moreover, since the microstructure of the sintered body can be a composite structure made up of a complete alloy phase and a partially concentrated phase, the strength of the sintered body can be further improved. The partially diffusion-alloyed steel powder has therefore been widely developed as it can meet the recent requests for strengthening parts.

Basic alloy components typically used in the production of the partially diffusion-alloyed steel powder include Ni and Mo.

Ni has an effect of improving the toughness of the sintered body. By adding Ni, austenite is stabilized, as a result of which more austenite remains as retained austenite without transforming into martensite after quenching. Ni also has an effect of strengthening the matrix of the sintered body by solid solution strengthening.

Mo has an effect of improving hardenability. Thus, Mo suppresses the formation of ferrite during quenching and facilitates the formation of bainite or martensite, to transformation-strengthen the matrix of the sintered body. Mo also has both an effect of solid solution strengthening by dissolving in the matrix and an effect of strengthening the matrix by precipitation by forming fine carbides. Furthermore, Mo has good gas carburizing property and is a non-grain boundary oxidizable element, and so has an effect of strengthening the sintered body by carburizing.

As an example of a mixed powder for high strength sintered parts using a partially diffusion-alloyed steel powder containing these alloy components, JP 3663929 B (PTL 1) discloses a mixed powder for high strength sintered parts yielded by mixing an alloyed steel powder obtained by partially alloying Ni: 0.5 mass % to 4 mass % and Mo: 0.5 mass % to 5 mass %, with Ni: 1 mass % to 5 mass %, Cu: 0.5 mass % to 4 mass %, and a graphite powder: 0.2 mass % to 0.9 mass %.

As an example of an iron-based sintered body having high density and not containing Ni, JP H4-285141 A (PTL 2) discloses a method of producing an iron-based sintered body by mixing an iron-based powder of 1 μm to 18 μm in average particle size with a Cu powder of 1 μm to 18 μm in average particle size at a weight ratio of 100:(0.2 to 5) and forming and sintering the mixed powder. The technique described in PTL 2 uses an iron-based powder having an extremely smaller average particle size than a typical iron-based powder, and thus achieves a high sintered body density of 7.42 g/cm³ or more which is normally impossible.

As an example of a mixed powder for high strength sintered parts using a partially diffusion-alloyed steel powder, JP 4483595 B (PTL 3) discloses a mixed powder for high strength sintered parts obtained by mixing an alloyed steel powder having diffusionally adhering Ni and Mo, with a metal Cu powder and a graphite powder.

CITATION LIST Patent Literatures

PTL 1: JP 3663929 B

PTL 2: JP H4-285141 A

PTL 3: JP 4483595 B

SUMMARY Technical Problem

However, we found out as a result of study that the sintered materials produced using the respective mixed powders described in PTL 1 and PTL 3 and the sintered material produced by the method described in PTL 2 have the following problems.

The sintered material described in PTL 1 requires at least 1.5 mass % Ni, and substantially contains 3 mass % or more Ni as can be understood from its Example. Thus, a large amount of Ni, e.g. 3 mass % or more, is needed in order to achieve a high strength of 800 MPa or more in the sintered material described in PTL 1. A larger amount of Ni is likely to be needed in order to obtain a sintered body having a strength of 1000 MPa or more after carburizing, quenching, and tempering.

Ni, however, is an unfavorable element in terms of environmental responsiveness and recyclability in recent years, and it is desirable to avoid using Ni as much as possible. Adding a few mass % Ni is also very disadvantageous in terms of cost. Besides, the use of Ni as an alloying element requires prolonged sintering in order to sufficiently diffuse Ni in the iron powder or alloyed steel powder.

The sintered material described in PTL 2 contains no Ni, but the average particle size of the iron-based powder used is 1 μm to 18 μm which is smaller than normal. Such a small particle size causes lower fluidity of the mixed powder, and decreases work efficiency when filling the die with the powder in press forming.

The sintered material described in PTL 3 contains a metal Cu powder in the mixed powder. The metal Cu powder melts and permeates between the particles of the iron powder during sintering, and thus increases the distance between the particles of the iron powder. This causes the size of the sintered body to be larger than the size of the green compact. Consequently, the density of the sintered body is lower than the density of the green compact. This phenomenon is commonly known as Cu growth. A significant decrease in density caused by the Cu growth leads to drawbacks such as lower strength and toughness of the sintered body.

It could therefore be helpful to provide a method of producing a mixed powder for powder metallurgy that, despite not containing Ni (Ni-free), enables the production of a sintered body having excellent properties (e.g. tensile strength and toughness after carburizing, quenching, and tempering) at least as high as those in the case of containing Ni. It could also be helpful to provide a method of producing a sintered body using the mixed powder for powder metallurgy, and a sintered body obtained by the production method.

Solution to Problem

We conducted various studies on alloy components of a mixed powder for powder metallurgy not containing Ni and means for adding the alloy components. As a result, we made the following discoveries (1) to (6).

(1) In some cases, by using a mixed powder for powder metallurgy obtained by partially diffusing Mo and Cu in an iron-based powder to yield a partially diffusion-alloyed steel powder beforehand and mixing the partially diffusion-alloyed steel powder with a graphite powder, a sintered body having properties at least as high as those in the case of containing Ni is obtained, despite not containing Ni.

(2) In the cases of (1), Mo functions as a ferrite-stabilizing element during sintering. Hence, ferrite phase forms in a portion having a large amount of Mo and its vicinity to facilitate the sintering of the iron powder, as a result of which the density of the sintered body increases.

(3) In the cases of (1), when carburizing and quenching the sintered body, Cu shifts the martensite transformation start temperature to the lower temperature side, thus strengthening the sintered body.

(4) In the cases of (1), to obtain a sintered body having excellent properties, it is necessary to limit the chemical composition of the mixed powder for powder metallurgy to a specific range, set the average particle size of the iron-based powder to 30 μm to 120 μm, and use not a metal Cu powder but a cuprous oxide (Cu₂O) powder as a Cu source when producing the partially diffusion-alloyed steel powder.

(5) The use of a cuprous oxide powder can prevent Cu growth which occurs in the case of using a metal Cu powder, and suppress a decrease in the density of the sintered body.

(6) The use of an iron-based powder having an average particle size of 30 μm to 120 μm can improve the fluidity of the mixed powder for powder metallurgy.

This disclosure is based on the above-mentioned discoveries. We thus provide the following.

1. A method of producing a mixed powder for powder metallurgy, the method comprising: mixing an iron-based powder with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder; heat-treating the raw material mixed powder to cause Mo and Cu to diffusionally adhere to a surface of the iron-based powder, to obtain a partially diffusion-alloyed steel powder; and mixing the partially diffusion-alloyed steel powder with a graphite powder, to obtain a mixed powder for powder metallurgy, wherein the iron-based powder has an average particle size of 30 μm to 120 μm, a cuprous oxide powder is used as the Cu-containing powder, and the mixed powder for powder metallurgy has a chemical composition containing (consisting of) Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %, with a balance being Fe and incidental impurities.

2. The method of producing a mixed powder for powder metallurgy according to 1., wherein the Cu-containing powder has an average particle size of 5 μm or less.

3. A method of producing a sintered body, the method comprising: forming a mixed powder for powder metallurgy obtained by the method of producing a mixed powder for powder metallurgy according to 1. or 2.; and sintering the formed mixed powder.

4. A sintered body obtained by the method of producing a sintered body according to 3.

Advantageous Effect

It is possible to obtain a mixed powder for powder metallurgy that, despite not containing Ni, enables the production of a sintered body having excellent properties at least as high as those in the case of containing Ni. The mixed powder for powder metallurgy has high fluidity, and so contributes to excellent work efficiency when charging the mixed powder for powder metallurgy into a die for press forming. Moreover, a sintered body having both excellent strength and excellent toughness can be produced at low cost, even with an ordinary sintering method.

DETAILED DESCRIPTION

One of the disclosed embodiments is described in detail below.

A method of producing a mixed powder for powder metallurgy according to one of the disclosed embodiments includes the following (1) to (3):

(1) a first mixing step of mixing an iron-based powder with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder;

(2) a diffusional adhesion step of heat-treating the raw material mixed powder to cause Mo and Cu to diffusionally adhere to the surface of the iron-based powder, to obtain a partially diffusion-alloyed steel powder; and

(3) a second mixing step of mixing the partially diffusion-alloyed steel powder with a graphite powder, to obtain a mixed powder for powder metallurgy.

As the iron-based powder, an iron-based powder having an average particle size of 30 μm to 120 μm is used. As the Cu-containing powder, a cuprous oxide powder is used. The mixed powder for powder metallurgy has a chemical composition containing Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %, with the balance being Fe and incidental impurities.

With the conventional methods, a partially diffusion-alloyed steel powder having diffusionally adhering Mo is mixed with a metal Cu powder and a graphite powder to produce a mixed powder for powder metallurgy, as mentioned above. With the method according to this disclosure, on the other hand, Cu is caused to diffusionally adhere to an iron-based powder together with Mo beforehand, and a cuprous oxide powder is used as a Cu source for diffusional adhesion of Cu.

Each of the steps (1) to (3) is described below. In the following description, “%” denotes mass % unless otherwise noted. Mo content, Cu content, and graphite powder content denote the contents of the respective components in the whole mixed powder for powder metallurgy.

[First Mixing Step]

In the first mixing step, an iron-based powder is mixed with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder. The mixing method used in the first mixing step is not limited, and may be a conventional method using a Henschel mixer, a cone mixer or the like. The mixing ratio of the iron-based powder, the Mo-containing powder, and the Cu-containing powder is adjusted so that the resulting mixed powder for powder metallurgy has a chemical composition in the below-mentioned range. In detail, the powders are mixed so that the Mo content is 0.2% to 1.5% and the Cu content is 0.5% to 4.0% with respect to the whole mixed powder for powder metallurgy.

(Iron-Based Powder)

Average particle size: 30 μm to 120 μm

In this disclosure, the average particle size of the iron-based powder is 30 μm to 120 μm. If the average particle size of the iron-based powder is less than 30 μm, the iron-based powder itself or the raw material mixed powder obtained using the iron-based powder decreases in fluidity, which causes a decrease in work efficiency such as die filling. The average particle size of the iron-based powder is therefore 30 μm or more. The average particle size is preferably 40 μm or more, and more preferably 50 μm or more. If the average particle size of the iron-based powder is more than 120 μm, the driving force for density enhancement during sintering decreases, and coarse holes are formed around coarse iron powder particles, causing a decrease in the density of the sintered body. This leads to lower strength and toughness of the sintered body. The average particle size of the iron-based powder is therefore 120 μm or less. The average particle size is preferably 100 μm or less, and more preferably 80 μm or less. In this disclosure, the term “average particle size” denotes volume-based median size (d₅₀).

The term “iron-based powder” denotes a powder whose Fe content is 50 mass % or more. Examples of the iron-based powder include a pure iron powder and an alloyed steel powder. As the iron-based powder, an iron powder (pure iron powder) is preferable.

The method of producing the iron-based powder is not limited, and may be any method. In terms of availability, an iron-based powder produced by an atomizing method or a reduction method is preferable. As an iron-based powder produced by an atomizing method, any of an as-atomized powder and an atomized powder may be used. The as-atomized powder is a powder obtained by atomizing molten steel and optionally drying and classifying the resulting powder without heat treatment for deoxidation (reduction), decarburization, or the like. The atomized powder is a powder obtained by reducing an as-atomized powder through treatment in a reducing atmosphere. As an iron-based powder produced by a reduction method, a reduced iron powder obtained by reducing mill scale generated during production of steel materials or iron ore is preferable.

The apparent density of the iron-based powder is not limited, but is preferably 1.7 Mg/m³ to 3.5 Mg/m³. In the case of using an iron-based powder produced by an atomizing method as the iron-based powder, the apparent density of the iron-based powder is preferably about 2.0 Mg/m³ to 3.5 Mg/m³, and more preferably 2.5 Mg/m³ to 3.2 Mg/m³. In the case of using an iron-based powder produced by a reduction method as the iron-based powder, the apparent density of the iron-based powder is preferably about 1.7 Mg/m³ to 3.0 Mg/m³, and more preferably 2.2 Mg/m³ to 2.8 Mg/m³. The apparent density mentioned here is measured by the test method of JIS Z 2504.

The specific surface area of the iron-based powder is not limited, but is preferably 0.002 m²/g to 0.5 m²/g. In the case of using an iron-based powder produced by an atomizing method as the iron-based powder, the specific surface area of the iron-based powder is preferably about 0.005 m²/g or more, and more preferably 0.01 m²/g or more. The upper limit of the specific surface area is preferably 0.1 m²/g. In the case of using an iron-based powder produced by a reduction method as the iron-based powder, the specific surface area of the iron-based powder is preferably about 0.01 m²/g or more, and more preferably 0.02 m²/g or more. The upper limit of the specific surface area is preferably 0.3 m²/g.

(Mo-Containing Powder)

The Mo-containing powder is a powder that functions as a Mo source in the below-mentioned diffusional adhesion step. The Mo-containing powder may be any powder that contains Mo as an element. Hence, any of a metal Mo powder (pure Mo powder), a Mo alloyed powder, and a Mo compound powder may be used. As the Mo alloyed powder, for example, Fe—Mo (ferromolybdenum) powder may be used. As the Mo compound powder, a powder of a Mo compound such as Mo oxide, Mo carbide, Mo sulfide, or Mo nitride may be used. These Mo-containing powders may be used singly or in combination of two or more.

(Cu-Containing Powder)

The Cu-containing powder is a powder that functions as a Cu source in the below-mentioned diffusional adhesion step. In this disclosure, it is important to use a cuprous oxide powder as the Cu-containing powder. The cuprous oxide powder is reduced to metal Cu in the diffusional adhesion step, so that a partially diffusion-alloyed steel powder with Mo and Cu diffusionally adhering to the surface of the iron-based powder can be obtained.

By using the cuprous oxide powder as the Cu-containing powder, Cu growth which occurs in the case of using a metal Cu powder can be prevented to suppress a decrease in the density of the sintered body. Moreover, cuprous oxide is chemically stable and does not oxidize (rust) like metal Cu, and so can be handled easily. Furthermore, cuprous oxide (Cu₂O) is lower in oxidation number than copper oxide (CuO), and so can be easily reduced to metal Cu in the diffusional adhesion step. For example, in the case of heat-treating the raw material mixed powder in a hydrogen atmosphere in the diffusional adhesion step, the use of cuprous oxide can not only decrease the amount of hydrogen necessary for reduction but also lower the heating temperature, and further shorten the treatment time.

The average particle size of the Cu-containing powder is not limited, but is preferably 5 μm or less. An average particle size of 5 μm or less can further enhance the effect of improving strength and toughness by Cu. The average particle size is more preferably 4.5 μm or less. No lower limit is placed on the average particle size of the Cu-containing powder, yet excessively decreasing the average particle size causes an increase in the production cost of the Cu-containing powder. Accordingly, the average particle size of the Cu-containing powder is preferably 0.2 μm or more, and more preferably 1.0 μm or more.

In the case of conventionally used metal Cu powders, the average particle size of a typical commercially-available metal Cu powder is about 20 μm to 40 μm.

[Diffusional Adhesion Step]

The raw material mixed powder is then heat-treated. As a result of the heat treatment, Mo in the Mo-containing powder and Cu in the Cu-containing powder diffuse into the iron-based powder on the contact surface of the iron-based powder and the Mo-containing powder and the contact surface of the iron-based powder and the Cu-containing powder. A partially diffusion-alloyed steel powder with Mo and Cu diffusionally adhering to the surface of the iron-based powder is thus obtained.

The heat treatment may be performed in any atmosphere, but a reducing atmosphere is preferable, and a hydrogen-containing atmosphere is more preferable. As the hydrogen-containing atmosphere, a hydrogen gas atmosphere may be used. The heat treatment may be performed at atmospheric pressure, under reduced pressure, or under vacuum.

The temperature of the heat treatment is not limited, but is preferably 800° C. to 1000° C.

[Grinding and Classification]

Iron-based powder particles contained in the partially diffusion-alloyed steel powder obtained in this way are usually agglomerated as a result of sintering. It is therefore preferable to perform a grinding and classification step of grinding and classifying the partially diffusion-alloyed steel powder, after the diffusional adhesion step and before the subsequent second mixing step. For example, after grinding the partially diffusion-alloyed steel powder into a desired particle size, the partially diffusion-alloyed steel powder is classified using a sieve with a predetermined opening, thus removing a coarse powder. The maximum particle size of the partially diffusion-alloyed steel powder is preferably 180 μm or less.

The partially diffusion-alloyed steel powder may be optionally annealed, before the subsequent second mixing step.

The partially diffusion-alloyed steel powder preferably has a chemical composition containing Mo and Cu with the balance being Fe and incidental impurities. Examples of the incidental impurities contained in the partially diffusion-alloyed steel powder include C, O, N, and S. The contents of these elements are preferably C: 0.02% or less, 0: 0.3% or less, N: 0.004% or less, and S: 0.03% or less with respect to the total mass of the partially alloyed steel powder including the impurities. In particular, the 0 content is more preferably 0.25% or less. If the contents of the incidental impurities exceed these ranges, the mixed powder for powder metallurgy obtained as a result of the second mixing step decreases in compressibility, and is difficult to be compression molded into a green compact having sufficient density.

[Second Mixing Step]

The partially diffusion-alloyed steel powder obtained in this way is then mixed with a graphite powder, to yield a mixed powder for powder metallurgy. C which is a main component of the graphite powder has an effect of increasing the strength of the sintered body through, for example, strengthening by precipitation of carbides and hardenability enhancement. The addition of graphite is therefore essential, particularly to achieve a high tensile strength of 1000 MPa or more after carburizing, quenching, and tempering the sintered body.

The graphite powder may be mixed, for example, according to a conventional method typically used for powder mixing. The mixing ratio of the partially diffusion-alloyed steel powder and the graphite powder is adjusted so that the resulting mixed powder for powder metallurgy has a chemical composition in the below-mentioned range. In detail, the mixing is performed so that the C content is 0.1% to 1.0% with respect to the whole mixed powder for powder metallurgy.

(Graphite Powder)

The graphite powder is not limited, and may be any graphite powder. The average particle size of the graphite powder is not limited, but is preferably about 1 μm to 50 μm.

[Chemical Composition of Mixed Powder]

In this disclosure, the resulting mixed powder for powder metallurgy has a chemical composition containing Mo: 0.2% to 1.5%, Cu: 0.5% to 4.0%, and C: 0.1% to 1.0%, with the balance being Fe and incidental impurities. Although additives such as a lubricant may be added to the alloyed steel powder for powder metallurgy as described later, the “chemical composition of the mixed powder for powder metallurgy” mentioned here denotes the chemical composition of the mixed powder except the additives, i.e. the part composed of the partially diffusion-alloyed steel powder and the graphite powder.

The reasons for limiting the chemical composition of the mixed powder are given below.

Mo: 0.2% to 1.5%

If the Mo content is less than 0.2%, the effect of improving hardenability and the effect of improving the strength of the sintered body are insufficient. The Mo content is therefore 0.2% or more. The Mo content is preferably 0.3% or more, and more preferably 0.4% or more. If the Mo content is more than 1.5%, the hardenability improving effect is saturated, and the non-uniformity of the microstructure of the sintered body rather increases, causing a decrease in the strength and toughness of the sintered body. The Mo content is therefore 1.5% or less. The Mo content is preferably 1.0% or less, and more preferably 0.8% or less.

Cu: 0.5% to 4.0%

If the Cu content is less than 0.5%, the effects of solid solution strengthening and hardenability improvement by Cu are insufficient, and the strength and toughness of the sintered part decreases. The Cu content is therefore 0.5% or more. The Cu content is preferably 1.0% or more, and more preferably 1.5% or more. If the Cu content is more than 4.0%, the effect of improving the strength of the sintered part is saturated. The Cu content is therefore 4.0% or less. The Cu content is preferably 3.0% or less, and more preferably 2.5% or less.

C: 0.1% to 1.0%

C is an element having an effect of improving the strength and fatigue strength of the sintered body. To achieve this effect, the C content is 0.1% or more. If the C content is more than 1.0%, the material becomes hypereutectoid, and a lot of cementite precipitates and causes a decrease in the strength of the sintered body. The C content is therefore 1.0% or less.

A method of producing a sintered body according to one of the disclosed embodiments is described below. In this disclosure, a sintered body can be yielded by forming the above-mentioned mixed powder for powder metallurgy and sintering the formed mixed powder.

[Forming]

The forming is not limited, and may be performed by any method that can form the mixed powder for powder metallurgy. A typical forming method is a method of charging the mixed powder for powder metallurgy into a die and pressing it. The pressing force in the pressing is preferably 400 MPa to 1000 MPa. If the pressing force is less than 400 MPa, the density of the obtained green compact is low, and the properties of the sintered body decrease. If the pressing force is more than 1000 MPa, the life of the die shortens extremely, which is economically disadvantageous. The temperature in the pressing is preferably room temperature (about 20° C.) to about 160° C.

In the case where the resulting sintered body needs to be subjected to machining work or the like to shape the part, a powder for improving machinability may be added to the mixed powder for powder metallurgy before the forming. For example, MnS and the like may be used as the powder for improving machinability. The powder for improving machinability may be added according to a conventional method.

A lubricant may be added to the mixed powder for powder metallurgy, before the forming. As the lubricant, a lubricant powder is preferable. Alternatively, the forming may be performed with the lubricant being applied or adhered to the die. In either case, any lubricant such as metal soap, e.g. zinc stearate or lithium stearate, amide-based wax, e.g. ethylenebisstearamide, may be used as the lubricant. In the case of mixing the lubricant into the mixed powder for powder metallurgy, the amount of the lubricant is preferably about 0.1 parts by mass to 1.2 parts by mass with respect to 100 parts by mass the mixed powder for powder metallurgy.

[Sintering]

The green compact obtained in this way is then sintered. The sintering is preferably performed in a temperature range of 1100° C. to 1300° C. If the sintering temperature is less than 1100° C., the sintering does not progress sufficiently, and a sintered body having excellent tensile strength (1000 MPa or more) is hard to be obtained. If the sintering temperature is more than 1300° C., the life of the sintering furnace shortens, which is economically disadvantageous. The sintering time is preferably 10 minutes to 180 minutes.

A sintered body obtained using the mixed powder for powder metallurgy according to this disclosure by the above-mentioned procedure has excellent tensile strength and toughness, as compared with a sintered body obtained by forming and sintering a conventional powder under the same conditions.

The obtained sintered body may be optionally subjected to strengthening treatment. Examples of the strengthening treatment include carburizing-quenching, bright quenching, induction hardening, and carburizing nitriding treatment. Even in the case where such strengthening treatment is not performed, the sintered body produced using the mixed powder for powder metallurgy according to this disclosure has improved strength and toughness as compared with a conventional sintered body not subjected to strengthening treatment. Each strengthening treatment may be performed according to a conventional method.

EXAMPLES

More detailed description is given below, based on examples. This disclosure is, however, not limited to these examples.

Each mixed powder for powder metallurgy was produced by the following procedure.

(First Mixing Step)

An iron-based powder was mixed with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder. As the iron-based powder, an as-atomized powder having the apparent density listed in Table 1 was used. The specific surface area of the iron-based powder was 0.39 m²/g. As the Mo-containing powder, an oxidized Mo powder having an average particle size of 10 μm was used. As the Cu-containing powder, a cuprous oxide powder having the average particle size listed in Table 1 was used. The mixing was performed for 15 minutes using a V-shaped mixer. The amount of each powder was adjusted so that the Mo and Cu contents in the resulting mixed powder for powder metallurgy were as listed in Table 1.

(Diffusional Adhesion Step)

Next, the obtained raw material mixed powder was heat-treated to cause Mo and Cu to diffusionally adhere to the surface of the iron-based powder, thus yielding a partially diffusion-alloyed steel powder. The heat treatment was performed at a temperature of 880° C. for 1 hour in a hydrogen atmosphere with a dew point of 30° C.

In some Comparative Examples (Nos. 1 and 3), instead of adding the Cu-containing powder in the first mixing step, a metal Cu powder was added in the second mixing step, and only the Mo-containing powder was added in the first mixing step and then the diffusional adhesion step was performed. For comparison, in No. 29, the iron-based powder was mixed with a metal Ni powder having an average particle size of 8 μm, a metal Cu powder (the same metal Cu powder as in Comparative Examples Nos. 1 and 3) having an average particle size of 28 μm, and an oxidized Mo powder (the same oxidized Mo powder as in Examples) having an average particle size of 10 μm in the first mixing step, and then subjected to the diffusional adhesion step. The composition of the partially diffusion-alloyed steel powder in No. 29 was 4% Ni—1.5% Cu—0.5 Mo—Fe.

(Grinding and Classification)

The obtained partially diffusion-alloyed steel powder was ground and classified by the following procedure. First, since the partially diffusion-alloyed steel powder tends to agglomerate due to the heat treatment, the partially diffusion-alloyed steel powder was ground three times using a hammer mill. Here, the grate opening of the hammer mill was sequentially reduced as follows: 3 mm (first time), 2 mm (second time), and 1 mm (third time). The ground powder was then put through a vibrating sieve with an opening of 180 μm, and only parts with a grain size of 180 μm or less passing the sieve were collected while removing/discarding a coarse powder remaining on the sieve. The obtained powder was then subjected to the subsequent second mixing step.

(Second Mixing Step)

Next, a graphite powder (average particle size: 5 μm) of the content listed in Table 1 was added to the partially diffusion-alloyed steel powder. 0.6 parts by mass ethylenebisstearamide was further added with respect to 100 parts by mass the mixed powder for powder metallurgy, and the resulting powder was mixed for 15 minutes in a V-shaped mixer.

The balance of the alloyed steel powder in the table is iron and incidental impurities. The amount of incidental impurities in the alloyed steel powder used in this disclosure was 0.2% or less with respect to the partially diffusion-alloyed steel powder. In Nos. 1 and 3, a metal Cu powder was mixed together with the graphite powder so that the mixed powder for powder metallurgy had the Cu content listed in Table 1.

(Forming)

After this, the mixed powder for powder metallurgy was pressed to produce a rod-shaped green compact with a length of 55 mm, a width of 10 mm, and a thickness of 10 mm. Ten green compacts were made from each mixed powder for powder metallurgy. The density of the green compact was 7.0 Mg/m³.

(Sintering)

Each rod-shaped green compact was sintered to obtain a rod-shaped sintered body. The sintering was performed in a propane converted gas atmosphere which is a reducing atmosphere, at a temperature of 1130° C. for 20 minutes.

The properties of the mixed powder for powder metallurgy and the rod-shaped sintered body obtained by the above-mentioned procedure were then evaluated by the following methods. The results are listed in Table 1.

(Fluidity of Mixed Powder for Powder Metallurgy)

100 g of the test powder was collected from the mixed powder for powder metallurgy, and passed through a nozzle of 5 mmφ. The case where the powder flew through the nozzle completely without stopping was rated as “pass”, and the case where a part or whole of the powder stopped and did not flow through the nozzle was rated as “fail”.

(Tensile Strength)

A total of five tensile test pieces, i.e. one from each of five rod-shaped sintered bodies out of the ten rod-shaped sintered bodies, were cut out. Each tensile test piece had a parallel portion diameter of 5 mm and a gauge length of 15 mm. Each obtained tensile test piece was sequentially subjected to gas carburizing, quenching, and tempering under the following conditions:

-   -   gas carburizing: carbon potential: 0.8 mass %, temperature: 870°         C., time: 60 minutes,     -   quenching: temperature: 60° C., oil quenching,     -   tempering: temperature: 180° C., time: 60 minutes.

Using the tensile test piece obtained by this procedure, a tensile test was conducted by the method defined in JIS Z 2241, to measure the tensile strength. An average of the measurement values of the five test pieces was set as the tensile strength. The case where the measured tensile strength was 1000 MPa or more was rated as “pass”, and the case where the measured tensile strength was less than 1000 MPa was rated as “fail”.

(Toughness)

To evaluate the toughness of the sintered body, a Charpy impact test was conducted. In the Charpy impact test, the remaining five rod-shaped sintered bodies out of the ten rod-shaped sintered bodies were used as test pieces without changing their shape, and the impact value was measured by the method defined in JIS Z 2242. Before the Charpy impact test, each of the rod-shaped sintered bodies was subjected to gas carburizing, quenching, and tempering under the same conditions as the above-mentioned tensile test pieces. An average of the measurement values of the five test pieces was set as the impact value. The case where the measured impact value was 14.5 J/cm² or more was rated as “pass”, and the case where the measured impact value was less than 14.5 J/cm² was rated as “fail”.

TABLE 1 Production conditions First mixing step (partially diffusion-alloyed steel powder) Second mixing step Iron-based powder Cuprous oxide powder Metal Cu powder Graphite Average Average Content of alloying element Average powder Apparent density particle size particle size Ni*¹ Mo*¹ Cu*¹ particle size Cu*¹ C*¹ No. (Mg/m³) (μm) (μm) (mass %) (mass %) (mass %) (μm) (mass %) (mass %) 1 3.10 60 — 0 0.4 0   28 2.0 0.3 2 3.10 60 4.2 0 0.4 2.0 — 0 0.3 3 3.10 60 — 0 0.8 0   28 2.0 0.3 4 3.10 60 2.5 0 0.8 2.0 — 0 0.3 5 2.50 20 3.1 0 0.6 2.0 — 0 0.3 6 2.80 40 3.1 0 0.6 2.0 — 0 0.3 7 2.90 50 3.1 0 0.6 2.0 — 0 0.3 8 2.95 80 3.1 0 0.6 2.0 — 0 0.3 9 3.00 100  3.1 0 0.6 2.0 — 0 0.3 10 3.00 150  3.1 0 0.6 2.0 — 0 0.3 11 3.10 60 3.1 0 0.1 2.0 — 0 0.3 12 3.10 60 3.1 0 0.2 2.0 — 0 0.3 13 3.10 60 3.1 0 0.4 2.0 — 0 0.3 14 3.10 60 3.1 0 0.6 2.0 — 0 0.3 15 3.10 60 3.1 0 0.8 2.0 — 0 0.3 16 3.10 60 3.1 0 1.5 2.0 — 0 0.3 17 3.10 60 3.1 0 2.0 2.0 — 0 0.3 18 3.10 60 3.1 0 0.6 0.2 — 0 0.3 19 3.10 60 3.1 0 0.6 0.5 — 0 0.3 20 3.10 60 3.1 0 0.6 1.5 — 0 0.3 21 3.10 60 3.1 0 0.6 3.0 — 0 0.3 22 3.10 60 3.1 0 0.6 4.0 — 0 0.3 23 3.10 60 3.1 0 0.6 5.0 — 0 0.3 24 3.10 60 3.1 0 0.6 2.0 — 0  0.05 25 3.10 60 3.1 0 0.6 2.0 — 0 0.2 26 3.10 60 3.1 0 0.6 2.0 — 0 0.5 27 3.10 60 3.1 0 0.6 2.0 — 0 1.0 28 3.10 60 3.1 0 0.6 2.0 — 0 1.5 29 2.80 65 —*² 4 0.5 1.5 — 0 0.3 30 3.10 60 0.2 0 0.4 2.0 — 0 0.3 31 3.10 60 9   0 0.4 2.0 — 0 0.3 Evaluation results Sintered body Partially diffusion-alloyed Strength Toughness steel powder Tensile strength Impact value No. Fluidity (MPa) Pass/fail (J/cm²) Pass/fail Remarks  1 Pass 1050 Pass 14.2 Fail Comparative Example  2 Pass 1070 Pass 14.9 Pass Example  3 Pass 1038 Pass 13.9 Fail Comparative Example  4 Pass 1065 Pass 14.6 Pass Example  5 Fail — — — — Comparative Example  6 Pass 1075 Pass 15.0 Pass Example  7 Pass 1070 Pass 14.9 Pass Example  8 Pass 1056 Pass 14.6 Pass Example  9 Pass 1041 Pass 14.5 Pass Example 10 Pass 1025 Pass 14.1 Fail Comparative Example 11 Pass 995 Fail 13.5 Fail Comparative Example 12 Pass 1032 Pass 14.5 Pass Example 13 Pass 1058 Pass 15.1 Pass Example 14 Pass 1068 Pass 14.7 Pass Example 15 Pass 1079 Pass 14.7 Pass Example 16 Pass 1052 Pass 14.5 Pass Example 17 Pass 998 Fail 14.0 Fail Comparative Example 18 Pass 1001 Pass 13.9 Fail Comparative Example 19 Pass 1025 Pass 14.5 Pass Example 20 Pass 1044 Pass 14.6 Pass Example 21 Pass 1072 Pass 14.7 Pass Example 22 Pass 1085 Pass 14.6 Pass Example 23 Pass 1110 Pass 14.2 Fail Comparative Example 24 Pass 992 Fail 14.8 Pass Comparative Example 25 Pass 1024 Pass 14.8 Pass Example 26 Pass 1075 Pass 14.7 Pass Example 27 Pass 1089 Pass 14.5 Pass Example 28 Pass 985 Fail 13.8 Fail Comparative Example 29 Pass 998 Fail 13.3 Fail Comparative Example 30 Pass 1060 Pass 15.5 Pass Example 31 Pass 1048 Pass 14.5 Pass Example *¹Content with respect to whole mixed powder for powder metallurgy, with balance being Fe and incidental impurities. *²Metal Cu powder added instead of cuprous oxide powder.

As is clear from the results in Table 1, in Examples satisfying the conditions according to this disclosure, a sintered body having tensile strength and toughness at least as high as those in Comparative Example (No. 29) using Ni was obtained, despite not using Ni. Moreover, the alloyed steel powder for powder metallurgy in these Examples had excellent fluidity. 

1. A method of producing a mixed powder for powder metallurgy, the method comprising: mixing an iron-based powder with a Mo-containing powder and a Cu-containing powder, to obtain a raw material mixed powder; heat-treating the raw material mixed powder to cause Mo and Cu to diffusionally adhere to a surface of the iron-based powder, to obtain a partially diffusion-alloyed steel powder; and mixing the partially diffusion-alloyed steel powder with a graphite powder, to obtain a mixed powder for powder metallurgy, wherein the iron-based powder has an average particle size of 30 μm to 120 μm, a cuprous oxide powder is used as the Cu-containing powder, and the mixed powder for powder metallurgy has a chemical composition containing Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %, with a balance being Fe and incidental impurities.
 2. The method of producing a mixed powder for powder metallurgy according to claim 1, wherein the Cu-containing powder has an average particle size of 5 μm or less.
 3. A method of producing a sintered body, the method comprising: forming a mixed powder for powder metallurgy obtained by the method of producing a mixed powder for powder metallurgy according to claim 1; and sintering the formed mixed powder.
 4. A sintered body obtained by the method of producing a sintered body according to claim
 3. 5. A method of producing a sintered body, the method comprising: forming a mixed powder for powder metallurgy obtained by the method of producing a mixed powder for powder metallurgy according to claim 2; and sintering the formed mixed powder.
 6. A sintered body obtained by the method of producing a sintered body according to claim
 5. 